Use a tether. Tie a rope to the trailer and wrap the free end around a tree.
Leave it loose as you back the trailer up by hand, but keep the loose end close
at hand. If the trailer starts to get away from you, just pull on the loose end
to tighten the rope around the tree.

This sounds like a practical idea. I don't have a tree nearby. But I
can pound a metal bar into the ground as an anchor point and wrap a
rope onto it. Nevertheless, the device that I am asking about probably
is easier to use.

Jay Chan